Injury and illness are athletes' sworn enemies.
However, sometimes, they can also be weapons in helping an athlete surmount previously unbreachable barriers.
Petra Kvitova, the world's fourth-ranked woman tennis player, is a shining example of this.
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GAINING A CRITICAL EDGE
But in deciding very close points - like key points, break points, set points, I think you really have to be mentally focused... because it helps in deciding matches.
PETRA KVITOVA, Czech tennis star
The two-time Wimbledon winner has had to endure a tough season as she was struck down by mononucleosis. It is a debilitating disease which can leave sufferers feeling fatigued for weeks, and even months.
The illness forced her to withdraw from multiple tournaments throughout the year.
Nonetheless, the Czech believes she has learnt valuable lessons from this trying period.
"It doesn't matter if the body part or the body isn't doing as well as you want it to be, you've just got to manage it better and that is probably the biggest thing I have learnt," she told The Straits Times in a phone interview.
"It is all mental."
Indeed, when she was fit enough to play this year, she was able to make several key breakthroughs.
First, she managed to retain a Women's Tennis Association title for the first time in her career when she won the Connecticut Open for the second straight year in August.
She reached the US Open quarter-finals for the first time too, her best showing in a Major which she admitted she had "always struggled with in the past".
But perhaps her most memorable breakthrough came at the Madrid Open in May. She finally registered a win over world No. 1 Serena Williams after five previous losses.
It was one of only six defeats suffered by the American this year.
"It was a superb result for me," said Kvitova.
"In tennis, everyone knows how to play the forehand and the backhand. But in deciding very close points - like key points, break points, set points, I think you really have to be mentally focused in the moment because it helps in deciding matches."
Williams has decided to skip the BNP Paribas WTA Finals Singapore presented by SC Global from Oct 23 to Nov 1 , citing health reasons for being unable to defend her crown.
Kvitova, who lies fifth on the Road To Singapore leaderboard, feels this could actually make things more interesting by making the playing field more open.
The Czech, who won the coveted season-ending tournament in 2011, said: "It's a little bit different when Serena is not playing because she is a great player and we all know who the biggest favourite is whenever she plays.
"Now, anyone can win it, and it's nice to see that every match can be very open."
With world No. 2 Simona Halep and No. 3 Maria Sharapova also struggling with injury, Kvitova's chances of winning a second WTA Finals title appear more than decent - assuming she qualifies for the eight-player competition.
Few who have seen her in full flight this season would doubt her.